Multi-site and bi-ventricular pacing are known in the art, primarily as methods for enhancing mechanical synchronization of the contraction of different parts of the heart muscle and reducing mitral regurgitation due to poorly-synchronized mechanical contractions. Such pacing is described, for example, in an article by S. Cazeau, et al., entitled "Multisite Pacing for End-Stage Heart Failure: Early Experience," in Pacing and Clinical Electrophysiology 19 (November, 1996), Part II, pages 1748-1757, which is incorporated herein by reference.
U.S. Pat. No. 5,800,464 to Kieval, which is incorporated herein by reference, describes an implantable system for effecting hyperpolarization of myocardial cells of a heart chamber, in order to enhance the relaxation thereof in the diastolic phase.
PCT patent application PCT/IL97/00012, published as WO 97/25098, to Ben-Haim et al., which is incorporated herein by reference, describes methods for modifying the force of contraction of at least a portion of a heart chamber by applying a non-excitatory electric field to the heart at a delay after electrical activation of the portion. The non-excitatory field is such as does not induce new activation potentials in cardiac muscle cells, but rather modifies the cells' response to the activation. In the context of the present patent application, the use of such a non-excitatory field is referred to as Excitable Tissue Control (ETC). The non-excitatory field may be applied in combination with a pacemaker or defibrillator, which applies an excitatory signal (i.e., pacing or defibrillation pulses) to the heart muscle.
PCT patent application PCT/IL97/00236, which is also incorporated herein by reference, describes a pacemaker that gives cardiac output enhancement. This pacemaker applies both excitatory (pacing) and non-excitatory (ETC) electrical stimulation pulses to the heart. By applying non-excitatory pulses of suitable strength, appropriately timed with respect to the heart's electrical activation, the contraction of selected segments of the heart muscle can be increased or decreased, thus increasing or decreasing the stroke volume of the heart. The PCT application suggests that pacing electrodes may be placed in two, three or all four chambers of the heart, in accordance with methods of multi-chamber pacing known in the art.